Drugs and other active agents are most frequently administered orally by means of solid dosage forms such as tablets and capsules. Large scale production methods used for their preparation usually require that they contain other materials in addition to the active ingredients. Additives may also be included in the formulations to facilitate handling, enhance the physical appearance, improve stability and aid in delivery of the drug to the bloodstream after administration.
Tablets are defined as solid pharmaceutical dosage forms containing drug substances with or without suitable diluents. Typically, each tablet contains a single dose of an effective amount of the pharmaceutical agent. Tablets are prepared by compression, extrusion or molding methods. Tablets are popular as a dosage form because of the advantages afforded both to the manufacturer (e.g., simplicity and economy of preparation, stability and convenience in packaging, shipping and dispensing) and the patient (e.g., accuracy of dosage, compactness, portability, blandness of taste and ease of administration). Tablets are the most common form of solid dose drug delivery. For review see, Pogany et al. (1988) Acta Pharm. Hung. 58:49-55; Doelker et al. (1988) Boll. Chim. Farm. 127:37-49; Hiestand et al. (1977) J. Pharm. Sci. 66:510-519; and Cooper et al. (1972) J. Pharm. Sci. 61:1511-1555.
Compressed tablets are usually prepared by large-scale production methods, while molded tablets generally involve small-scale operations. Compressed tablets usually contain no special coating and are made from a small number of powdered, crystalline or granular materials or "diluents," alone or in combination with disintegrants, controlled-release polymers, lubricants, diluents and, in many cases, colorants.
Compressed tablets may be coated with a variety of substances which may alter their physical characteristics. Sugar coated tablets contain a sugar coating which may be colored. Such coatings are beneficial in masking drugs possessing objectionable tastes or odors and in protecting materials sensitive to humidity, light or oxidation. Film-coated tablets are covered with a thin layer or film of water soluble or insoluble material. A number of polymeric substances with film forming properties can be used. Film coating imparts the same general characteristics as sugar coating with a much shorter period required for coating. Enteric coated tablets are coated with substances that resist dissolution in gastric fluid but disintegrate in the intestine. Enteric coatings are useful for tablets containing drugs that are inactivated or destroyed in the stomach, for those which irritate the mucosa or as a means of delayed release of the medication. Multiple compressed tablets are those made by more than one compression cycle. These include, layered tablets and press-coated tablets. Compressed tablets can be formulated into controlled-release tablets which release drug over a prolonged period of time to provide pulsatile or sustained release.
Compressed tablets can also be formed into tablets for purposes other than oral delivery. These include, but are not limited to, disintegration into solution, effervescent tablets, compressed suppositories or inserts, and buccal and sublingual tablets. Compressed tablets for preparing solutions include, for instance, Halzone Tablets for Solution and Potassium Permanganate Tablets for Solution. Effervescent tablets contain sodium bicarbonate and an organic acid such as tartaric or citric in addition to the drug. In the presence of water, these additives react to liberate carbon dioxide which acts as a disintegrator and provides effervescence. Sufficient acid is added to produce a neutral or slightly acidic reaction when disintegration in water is rapid and complete. Occasionally, vaginal suppositories, such as Metronidazole Tablets, are prepared by compression. Tablets for this use usually contain lactose as the diluent. Buccal and sublingual tablets are small, flat, oval tablets administered by insertion under the tongue or into the space between the cheek and gum where they dissolve slowly or erode; therefore, they are formulated and compressed with sufficient pressure to give a hard tablet. Progesterone or testosterone tablets may be administered in this manner.
There are a variety of methods of making compressed tablets. See, e.g., Remington: The Science and Practice of Pharmacy, Nineteenth Edition, Gennaro Ed. (1995) Vol. II pp. 1615-1649; Niwa et al. (1994) Pharm. Res. 11:478-484; and Franz et al. (1980) J. Pharm. Sci. 69:621-628. Interestingly, little has changed since the basic tableting method described in 1843. British Patent No. 9977. A number of parameters must be taken into account in tablet formulation such as moisture content and the physical characteristics of the substituents. This makes tablet formulation somewhat empirical. See, e.g., Lipps et al. (1994) J. Pharm. Sci. 83:937-947; Drissi-Alami et al. (1993) J. Pharm. Belg. 48:43-52; Ishino et al. (1990) Chem. Pharm. Bull. (Tokyo) 38:1987-1992; Sendall et al. (1986) J. Pharm. Pharmacol. 38:489-493; Fetzer et al. (1986) J. Pharm. Pharmacol. 38:254-258; Reading et al. (1984) J. Pharm. Pharmacol. 36:421-426; Paronen et al. (1983) J. Pharm. Pharmacol. 35:627-635; Nakagawa et al. (1982) Chem. Pharm. Bull. (Tokyo) 30:1401-1407; Elsabbagh et al. (1981) Pharmazie 36:488-492; Naito et al. (1977) J. Pharm. Sci. 66:254-259; Healey et al (1974) J. Pharm. Pharmacol. 26 Suppl: 41P-46P; and York et al. (1972) J. Pharm. Pharmacol. 24 Suppl:47P-56P.
A number of diluents are used in tableting to increase the bulk of the tablet to a practical size for compression. Diluents commonly used for this purpose include dicalcium phosphate dihydrate, tricalcium phosphate, calcium sulfate, lactose, spray-dried lactose, pregelatinized starch, microcrystalline cellulose, cellulose, kaolin, mannitol, sodium chloride, dry starch and powdered sugar. Certain diluents, particularly mannitol, lactose, sorbitol, sucrose and inositol, are used to make chewable tablets. Sucrose and mannitol are most frequently used in chewable tablets (AmStar and ICI Americas, respectively). Microcrystalline cellulose (Avicell, FMC) is a nonfibrous form of cellulose obtained by spray-drying washed, acid-treated cellulose and is available in several grades that range in average particle size from 20-100 .mu.m. Certain drawbacks are inherent in the use of any diluent and they must be chosen based on their intended use and reactivity with the drug. For instance, the combination of amine bases or amine salts with lactose in the presence of an alkaline lubricant, results in losses in the bioavailaibility of the active agent and tablets that discolor on aging.
The superior tableting properties of lactose have, however, resulted in its widespread use and intensive study as a tableting excipient. Sebhatu et al. (1994) Pharm. Res. 11:1233-1238; Vromans et al. (1985) Acta Pharm. Suec. 22:163-172; Vromans et al. (1985) Pharm Weekbl Sci! 7:186-193; De Boer et al. (1986) Pharm Weekbl Sci! 8:145-150; Van Kamp et al. (1986) Acta Pharm. Suec. 23:217-230; Vromans et al. (1986) Acta Pharm. Suec. 23:231-241; Georgakopoulos et al. (1983) 38:240-243; and York et al. (1973) J. Pharm. Pharmacol. 25:Suppl:1P-11P. Hydrous lactose does not flow and its use is limited to tablet formulations prepared by the wet granulation method. Both anhydrous lactose and spray-dried lactose have good flowability and compressibility and can be used in direct compression provided a suitable disintegrant and lubricant are present in the tablet. Other constraints to the use of lactose in tableting are discussed below.
Agents used to impart cohesive qualities to the powdered material are referred to as binders or granulators. They impart a cohesiveness to the tablet formulation which insures the tablet remaining intact after compression, as well as improving the free-flowing qualities by the formulation of granules of desired hardness and size.
The selection of a particular formulation of components for use in tableting is determined by a variety of parameters including the physical characteristics of the finished tablet. The exact formulation will contain a number of components, each chosen to impart a specific function and together to effect the specific properties desired in a tablet and is largely determined empirically. The physical characteristics of tablets are measured in terms of strength, friability uniformity of dimensions and disintegration time. Tablet strength, also termed hardness or tensile strength, is a measure of the cohesiveness of a tablet. Hardness is defined as the resistance of the tablet to chipping, abrasion or breakage under conditions of storage, transportation and handling. There are a number of machines manufactured for measuring hardness, such as the Hebelein, distributed by Vector. If a tablet is too hard, it may not disintegrate in the required period of time or meet the dissolution specification; if it is too soft, it will not withstand handling during subsequent processing, packaging, etc. Friability, is the ability of a tablet to resist abrasion. Friability is measured by tumbling tablets and determining the weight loss. Tumbling can be performed manually or mechanically, for instance by a Roche friabilator. The thickness, weight, disintegration time and content uniformity of a tablet must be relatively invariant from run to run. Tablets may be subject to further processing such as coating prior to packaging. A wide variety of coatings are known in the art.
Trehalose, (.alpha.-D-glucopyranosyl-.alpha.-D-glucopyranoside), is a naturally occurring, non-reducing disaccharide which was initially found to be associated with the prevention of desiccation damage in certain plants and animals which can dry out without damage and can revive when rehydrated. Trehalose is available commercially in the dihydrate form, having a 8-10% water content. Trehalose has been shown to be useful in preventing denaturation of proteins, viruses and foodstuffs during desiccation. See U.S. Pat. Nos. 4,891,319; 5,149,653; 5,026,566; Blakeley et al. (1990) Lancet 336:854-855; Roser (July 1991) Trends in Food Sci. and Tech. 166-169; Colaco et al. (1992) Biotechnol. Internat. 345-350; Roser (1991) BioPharm. 4:47-53; Colaco et al. (1992) Bio/Tech. 10:1007-1011; and Roser et al. (May 1993) New Scientist, pp. 25-28. Trehalose dihydrate is available in good manufacturing process (GMP) grade crystalline formulations. A method of making a desiccant, anhydrous form of trehalose is described in EP patent publication no. 600 730. This method involves heating a trehalose syrup in the presence of a seed crystal and recovering the anhydrous trehalose.
Trehalose is found extensively in such diverse animal and plant species as bacteria, yeast, fungi, insects and invertebrates. In many insects, it is the major blood sugar. The only major source for man is dietary in foods such as mushrooms and yeast products. Madsarovova-Nohejlova (1973) Gastroenterol. 65:130-133.
Trehalose is described for use in a peritoneal dialysis system in U.S. Pat. No. 4,879,280 where it is mentioned as one of several disaccharides as a replacement for the prior art system which utilized glucose. Trehalose is mentioned for use in the dialysis system as a disaccharide that will not be readily cleaved to glucose and thus avoid raising the blood glucose level. Trehalose has also been described as suitable for use in parenteral formulations primarily because it can be sterilized by autoclaving without the browning associated with conventional parenteral formulations. Japanese Patent No. 6-70718.
Trehalose is a common component of the human diet and information is available on its metabolism. Following oral ingestion, trehalose is not absorbed intact through the gastrointestinal tract, as only monosaccharides can pass throughout the intestinal epithelium. Ravich and Bayless (1983) Clin. Gast. 12:335-356. Trehalose is metabolized by the enzyme trehalase into two molecules of glucose. Sacktor (1968) Proc. Natl. Acad. Sci. USA 60:1007-1014. Trehalase is a normal constituent of most mammalian bodies, including humans, and has been identified in human serum, lymphocytes and the liver, but is principally located in the brush border of the intestinal tract and the renal proximal tubules. Belfiore et al. (1973) Clin. Chem. 19:447-452; Eze (1989) Biochem. Genet. 27:487-495; Yoshida et al. (1993) Clin. Chim. Acta 215:123-124; and Kramers and Catovsky (1978) Brit. J. Haematol. 38:4453-461. Trehalase is a membrane bound protein of the human and animal intestinal tract. Bergoz et al. (1981) Digestion 22:108-112; Riby and Galand (1985) Comp. Biochem. Physiol. 82B:821-827; and Chen et al. (1987) Biochem. J. 247:715-724.
The process by which intestinal trehalase metabolizes exogenous trehalose has been described. Intestinal hydrolases, such as trehalase, are surface components attached to the external side of the luminal membrane microvilli, and may be anchored to the membrane by phosphatidylinositol. Maestracci (1976) Biochim. et Biophys. Acta 433:469-481; and Galand (1989) Comp. Biochem. Physiol. 94B:1-11. Trehalose is hydrolyzed on the brush border surface of the enterocyte, where the two glucose molecules produced are released in close proximity to the membrane. Ravich and Bayless (1983). Glucose molecules are absorbed at the membrane by an active rather than a passive transport system. This physiological function was originally described in conjunction with other sugars as "membrane contact digestion."
U.S. Pat. Nos. 4,678,812 and 4,762,857 describe methods and compositions of tableting powders using the S-1 process of powder formation. These patents describe a process of forming an aqueous mixture of all the components of the finished tablet including trehalose, active ingredients, excipients, etc., spraying the aqueous mixture onto the surface of a moving bath of perfluorocarbon liquid, followed by lyophilization of the frozen droplets to dried powders. The S-1 method is used to prevent the formation of amorphous forms of trehalose. Also, the mixing of all the components in aqueous solution prior to formation of the powder an integral part of the claimed invention as it is required to achieve the necessary degree of homogeneity, that is, batch to batch standardization. The S-1 method is described more fully in U.S. Pat. Nos. 3,932,943 and 3,721,725. Trehalose is also described for use as a desiccant in food products, cosmetics and pharmaceuticals. EP publication nos. 606 753 A2; and 636 693.
All references cited herein, both supra and infra, are hereby incorporated herein by reference.